This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. We reported the FPOP (fast photochemical oxidation of proteins) method whereby the exposed amino-acid residues are covalently labeled by oxidation with hydroxyl radical produced by photolysis of hydrogen peroxide. Modified residues can be detected and quantified by standard trypsin proteolysis followed by LC/MS/MS analysis. It has been well established by the Chance group that 14 out of 20 amino acids can be modified by hydroxyl radical for the purpose of footprinting, thus potentially covering ~65% of the sequence of a typical protein. Since FPOP occurs in a time scale faster than protein conformational change, only the native conformation is labeled. Recently, sulfate radical anion as a new FPOP reagent was developed in our group. Being also a nonspecific labeling reagent, sulfate radical anion differs from OH radical in selectivity and reactivity, and is thus another possible tool for FPOP. Development of new FPOP reagents to label proteins in a more specific way is of great interests to us.